The present invention concerns a press roll, particularly useful in the paper industry, and particularly relates to means for adjusting the press roll against sag.
Press rolls of this kind are known from German Pat. No. 10 26 609 and from German Provisional Patent (Auslegeschrift) No. 22 54 392. The press roll, together with a parallel mating roll, defines a roll press which has a press nip between the two rolls. A web of material which is to be treated is passed though the nip. Such press rolls are used, for instance, at the wet end of paper making machines (in the pulp de-watering presses) or for the glazing of the dried paper, for instance in calenders. Particularly in calenders, extremely precise registry is required between the press roll and the mating roll in order that the thickness of the paper will be uniform over the width of the paper web. If the register between the press and mating rolls is inaccurate, then irregular paper rolls are produced when the web of paper is wound up. These irregular paper rolls are difficult to process further and lead to rejects.
For treating a web of paper in such a roll press, it may be desirable to exert different pressing forces on the marginal regions of the paper web, on the one hand, and on the central region of the web, on the other hand. This may be part of an attempt to eliminate irregularities in the thickness of the paper which may have been caused at other points in the paper making machine or process. Examples of such irregularities include irregular distribution of pulp weight per unit area, unequal moisture content of a dry felt web in the machine and unequal solids content of the paper web in front of a calender.
Known roll presses, including a press roll with adjustable sag and a solid mating roll, have the disadvantage that the desired uniformity in thickness of the treated web of material can frequently still not be obtained. Deviations still occur, which may differ in amount, depending on the value of the pressing force, and these are apparently due to the fact that the distance between the bearings of the mating roller is greater than that distance for the bearings of the shell of the press roll. (See the journal, "Das Papier" 1980, No. 4, pages 125-129). This disadvantage is inherent in all known press roll constructions, regardless of whether the rolls are of the so-called floating support type (German Pat. No. 10 26 609) or of the so-called hydrostatically supported type (German Provisional Pat. No. 22 54 392).
The journal "Das Papier" 1980, No. 5, pages 165-168 and 168-171, says that the irregularities in the thickness of the web can be reduced by means of additional measures. Nevertheless, the basic construction of roll presses has remained unchanged. Thus, it has not heretofore been possible to basically eliminate the causes of paper non-uniformity.